1. Field of the Invention
The present invention relates to a surface emitting semiconductor laser device (hereinafter referred to as surface emitting laser device) and, more particularly to a vertical-cavity surface emitting laser device that emits in a single transverse mode with excellent stability and is suited as a light source for use in the field of optical data transmission and optical communication.
2. Description of the Related Art
The surface emitting laser device, which emits laser light in a direction perpendicular to the substrate surface, attracts attention as a light source for use in the data communication field these days. One of the reasons for the attention is that a plurality of surface emitting laser devices can be arranged in a two-dimensional array on the same substrate, unlike the Fabry-Perot resonant cavity semiconductor laser device.
The surface emitting laser device has a pair of semiconductor multilayer reflectors (distributed Bragg reflector: DBR) each including AlxGa(1-x)As/AlyGa(1-y)As layer pairs (where x and y for the molecular ratio of GaAs and AlAs satisfy 0y less than x1), which overlie a semiconductor substrate made of GaAs or InP. The surface emitting laser device has between the pair of reflectors a vertical resonant cavity including an active layer structure and emits laser light in the direction perpendicular to the substrate surface.
In particular, the GaAs-group surface emitting laser device can employ DBRs including AlGaAs layers, which well lattice match with the GaAs substrate and have an excellent thermal conductivity and a higher reflectivity, and thus is expected for use as a laser device having an emission wavelength of 0.8 xcexcm to 1.0 xcexcm.
There is a current confinement structure available for the surface emitting laser device, in which a narrowed current injection area is provided to increase the current efficiency and decrease the threshold current of the laser device. The current confinement structure is categorized in two types: a current confinement structure having an ion-implanted p-n junction and an oxidized-Al current confinement structure. In the oxidized-Al current confinement structure, for example, the Al component in an AlAs or AlGaAs layer is selectively oxidized to have a peripheral oxidized-Al area and a central non-oxidized area, the latter constituting the current injection area. The oxidized-Al current confinement structure has an excellent current confinement function and can be fabricated relatively easily, whereby the oxidized-Al current confinement structure is widely used in the surface emitting laser device.
FIG. 1 is a perspective view illustrating the configuration of a GaAs-group surface emitting laser device having an oxidized-Al current confinement structure.
The surface emitting laser device 10 of FIG. 1 has a layer structure including an n-type lower DBR 14, a vertical resonant cavity 16, a p-type upper DBR 18, and a 10 nm-thick p-GaAs cap layer 20, which are deposited on an n-GaAs substrate 12.
The n-type lower DBR 14 is formed in a multi-layer reflector structure having 35 n-type Al0 2Ga0 8As/Al0.9Ga0 1As layer pairs.
The resonant cavity 16 includes an undoped Al0.3Ga0.7As lower cladding layer 16a, a GaAs/Al0.2Ga0 8As multi-quantum-well (MQW) active layer structure 16b, and an undoped Al0 3Ga0 7As upper cladding layer 16c. 
The p-type upper DBR 18 is formed in a multi-layer reflector structure having 20.5 p-type Al0.2Ga0 8As/Al0 9Ga0.1As layer pairs, with the bottom Al0 9Ga0 1As layer being replaced by a 50 nm-thick AlAs layer 24 to implement a current confinement structure.
In addition, the p-type cap layer 20, the p-type upper DBR 18, the resonant cavity 16, and the upper layers of the n-type lower DBR 14 are configured by etching to a mesa post 22.
For the current confinements structure, the AlAs layer 24 formed as the bottom layer of the p-type upper DBR 18 is selectively oxidized with steam at a high temperature from the periphery of the mesa post 22, thereby forming an annular oxidized-Al area 24B. The non-oxidized central area 24A of the AlAs layer 24 surrounded by the oxidized-Al area 24B serves as a current injection area.
A SiNx passivation film 26 is formed on the side-wall of the mesa post 22 and the n-type lower DBR 14 outside the mesa post 22. A polyimide layer 28 embeds the periphery of the mesa post 22 for achieving planarization, as well as for raising the thermal conductivity, reducing the parasitic capacitance, and improving the operating speed.
On top of the mesa post 22, there is provided an annular p-side electrode 30 in electric contact with the p-GaAs cap layer 20, whereas an n-side electrode 32 is provided on the bottom surface of the n-GaAs substrate 12.
FIGS. 2A to 2F depict the conventional surface emitting laser device of FIG. 1 during consecutive steps of fabrication thereof.
First, the n-GaAs substrate 12 is subjected to an acid treatment to clean the substrate surface, and then introduced into a MOCVD system, wherein 35 n-type Al0.2Ga0.8As/Al0 9Ga0.1As layer pairs are deposited by an epitaxial growth technique to form the n-type lower DBR 14 on the n-GaAs substrate 12. On the bottom layer of the p-type upper DBR 18, the 50 nm-thick AlAs film 25 is formed instead of the Al0.9Ga0.1As film.
Subsequently, the undoped Al0 3Ga0 7As cladding layer 16a, the GaAs/Al0.2Ga0.8As MQW active layer structure 16b, and the undoped Al0 3Ga0.7As cladding layer 16c are epitaxially deposited.
Thereafter, 20.5 p-type Al0.2Ga0 8As/Al0.9Ga0.1As layer pairs are stacked to form the p-type upper DBR 18, followed by epitaxial growth of the p-GaAs cap layer 20, thereby forming the layer structure as shown in FIG. 2A.
Subsequently, using a plasma CVD system, a SiNx film 33 is deposited on the p-GaAs cap layer 20. Further, a resist film (not shown) is deposited on the SiNx film 33, and then patterned by photolithography to form a resist mask 34 having a diameter of about 40 xcexcm, as shown in FIG. 2B.
After the resist mask 34 is formed, the SiNx film 33 is etched by reactive ion etching (RIE) using a CF4 gas as an etching gas and the resist mask 34 as an etching mask. Then, the p-type cap layer 20, the p-type upper DBR 18, the resonant cavity 16, and the top portion of the n-type lower DBR 14 are etched by a reactive ion beam etching (RIBE) system using a chlorine gas as an etching gas, to form a cylindrical mesa post 22, as shown in FIG. 2C.
After the etching is completed, the resist mask 34 is removed. Subsequently, the layer structure shown in FIG. 3C is subjected to a so-called wet oxidation treatment for about 25 minutes in a steam ambient at a temperature of 400.
As shown in FIG. 2D, the wet oxidation treatment causes the Al component in the AlAs layer 25 on the bottom of the p-type upper DBR 18 to be oxidized into Al2O3 from the outer periphery of the mesa post 22, thereby forming the oxidized-Al area 24B as a current confinement area at the bottom portion of the mesa post 22.
On the other hand, the central area of the AlAs layer 24 left as the non-oxidized area 24A serves as a current injection area. The current injection area 24A surrounded by the oxidized-Al area 24B is 5 xcexcm in diameter.
After the wet oxidation treatment is completed, the SiNx film 33 is removed by RIE.
Then, as shown in FIG. 2E, using a plasma CVD technique, the SiNx passivation film 26 is deposited on the top and side-wall of the mesa post 22 and on the n-type lower DBR 14 outside the mesa post 22.
The polyimide layer 28 is then formed on the SiNx passivation film 26 to bury the mesa post 22. Subsequently, by using a photolithographic technique, a portion of the polyimide layer 28 formed on top of the mesa post 22 is removed to expose the SiNx passivation film 26, as shown in FIG. 2E.
Thereafter, by a RIE technique using a CF4 gas as an etching gas, the SiNx passivation film 26 exposed on top of the mesa post 22 is selectively etched to form a window having a diameter of 30 xcexcm. Further, the annular p-side electrode 30 in electric contact with the p-type cap layer 20 is formed on the passivation film 26 by evaporation of AuZn, as shown in FIG. 2F.
After the p-side electrode 30 is formed, the bottom surface of the n-GaAs substrate 12 is polished to adjust the thickness of the substrate at 200 xcexcm. Then, an AuGeNi film is evaporated onto the bottom surface of the n-GaAs substrate to form the n-side electrode 32.
After the process as described above, a dicing saw is used for dicing the wafer mounting thereon a plurality of laser devices into a plurality of chips each including a laser device 10 as shown in FIG. 1.
In general, semiconductor laser devices used as light sources in the optical data transmission field emit laser in a single mode or a multimode.
A laser beam emitted in a single mode can transmit data at a higher speed compared to a laser beam emitted in a multimode. It is therefore desirable for the surface emitting laser device used as a light source in an optical data transmission system to emit either in a single-longitudinal mode or in a single-transverse mode.
The surface emitting laser device having the oxidized-Al current confinement structure inherently emits in a single-longitudinal mode due to the structure thereof. On the other hand, the difference in the refractive index between the non-oxidized area 24A as a current injection area and the oxidized-Al area 24 as a current confinement area may cause the surface emitting laser device to lase in a transverse mode. In addition, depending on the structure of the laser device, the lasing in the transverse mode may be sometimes a multimode lasing including a fundamental mode lasing and a higher-order mode lasing.
To implement the single-transverse mode lasing in the surface emitting laser device, the current injection area has typically a small area having a diameter of as small as 5 xcexcm in order to cut off the higher-order transverse mode lasings other than the fundamental transverse mode lasing.
However, there is a problem in the conventional method for forming the oxidized-Al current confinement structure, in which the Al component is selectively oxidized inwardly from the periphery of the mesa post to form the annular oxidized-Al area. That is, since it is difficult to precisely control the width of the oxidized-Al area of the current injection area in the conventional technique, higher-order transverse mode lasings other than the fundamental transverse mode lasing could not be necessarily removed as desired.
Consequently, the conventional surface emitting laser device having the oxidized-Al current confinement structure may lase in higher-order transverse modes, thereby making it difficult to lase in a single-transverse mode with high stability.
It is therefore an object of the present invention to provide a surface emitting laser device that lases in a single fundamental transverse mode with higher stability.
The present invention is directed to a surface emitting laser device having an oxidized-Al current confinement structure, formed on a substrate, and having a pair of semiconductor multilayer reflectors and a resonant cavity including an active layer disposed between the pair of semiconductor multilayer reflectors. The surface emitting laser device emits laser beams in a direction perpendicular to the substrate surface, wherein the cavity length is set in accordance with the relationship between the emission wavelength of the fundamental transverse mode of the laser device and the cavity length thereof at a predetermined temperature so that the resonant wavelength of the fundamental transverse mode is shorter than or equal to the peak-gain wavelength at a predetermined temperature, thereby allowing the fundamental transverse mode to have a maximum gain
In accordance with the surface emitting laser device of the present invention, it is possible to prevent emission in higher-order transverse modes, thereby allowing the surface emitting laser device to emit in a single fundamental transverse mode with higher stability.
To allow the resonant wavelength of the fundamental transverse mode to be shorter than or equal to the peak-gain wavelength at the predetermined temperature, for example, the cavity length is set so that the resonant wavelength of the fundamental transverse mode is shorter than or equal to the peak-gain wavelength at the predetermined temperature, in accordance with the relationship between the resonant wavelength of the fundamental transverse mode and the cavity length.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.